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Wear and Erosion in Large Caliber Gun Barrels
Richard G. Hasenbein
Weapon Systems & Technology Directorate
Armament Engineering & Technology Center
U.S. Army Armament Research, Development & Engineering Center
Mailing Address: Benét Laboratories
Watervliet Arsenal
Watervliet NY 12189-4050
[email protected]
PREFACE
“Wear and erosion” is one of several failure mechanisms that can cause large caliber Gun Barrels to be
condemned and removed from service. This paper describes the phenomenon, its causes and effects,
methods that are used to passively manage it, and steps that are taken to actively mitigate it.
1.0 GUN BARRELS – BACKGROUND
A large caliber Cannon (Figure 1) is a pressure vessel whose primary function is to accurately fire
projectiles at high velocities towards a target.
Figure 1: Representative Large Caliber Cannon
At its simplest, a Cannon consists of two major sub-assemblies:
• “Gun Barrel”: a long, slender Tube that serves multiple functions such as safely containing high
pressure combustion gases and providing a means for aiming/guiding the projectile in the intended
direction;
• “Breech”: an assembly that seals off the rear of the Gun Barrel during firing, but which can be
quickly opened to allow loading of ammunition. It also contains a device used to initiate the
combustion process.
Paper presented at the RTO AVT Specialists’ Meetingon “The Control and Reduction of Wear in Military Platforms”,
held in Williamsburg, USA, 7-9 June 2003, and published in RTO-MP-AVT-109.
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Wear and Erosion in Large Caliber Gun Barrels
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13. SUPPLEMENTARY NOTES
See also ADM201869, RTO-MP-AVT-109 The Control and Reduction of Wear in Military Platforms
(Contrôle et réduction de lusure des plates-formes militaires)., The original document contains color
images.
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Wear and Erosion in Large Caliber Gun Barrels
1.1 GUN BARREL INTERNAL GEOMETRY
Internally, a Gun Barrel often features three distinct regions (Figure 2):
• “bore”: a long cylindrical hole machined to exacting tolerances for diameter, axial straightness,
and surrounding wall thickness;
• “combustion chamber”: a much shorter hole at the breech-end of the Gun Barrel that is coaxial
with the bore and has a slightly larger diameter. It’s shape may be cylindrical, tapered, or both;
• “forcing cone”: a short tapered hole that connects the bore and combustion chamber, and is
coaxial with both.
(not to scale)
Figure 2: Gun Barrel Internal Geometry
The bores of Gun Barrels fall into two general categories: “smoothbore” and “rifled”. As the name
implies, the former type has an interior surface that is completely smooth and cylindrical, and the
projectiles that are fired from it usually have fins in order to achieve flight stability (similar to an arrow).
A rifled bore, on the other hand, contains many so-called “lands and grooves” that give the cross section a
castellated appearance (see section A-A in Figure 2 above). These lands and grooves are machined
axially, but with a slight twist or helix that causes the projectile to rotate while in-bore (see Figure 9).
This rotation continues after muzzle exit, providing gyroscopic flight stability to the projectile.
In this paper, the term “caliber” refers to the diameter of a Gun Barrel’s bore. In the specific case of a
rifled Barrel, this diameter is measured from the top of one internal land to that on the exact opposite side
of the bore.
NOTE: an alternative definition of the word “caliber” is the “total length of the Gun Barrel divided by the
diameter of its bore”. This gives a rough indication of the overall length-to-diameter (slenderness) ratio of
the Barrel. For example, a 155 millimeter (mm) Gun Barrel that is 6045 mm long would be referred to as
a 39-caliber Barrel (6045 divided by 155).
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1.2 MATING PROJECTILE GEOMETRY
Projectiles fired from large caliber Gun Barrels (Figure 3) are generally cylindrical, with aerodynamically-
shaped forward surfaces. The outside diameter is only slightly smaller than that of the bore, providing a
snug fit but allowing the projectile to slide in the axial direction within the bore. Most projectiles feature
one or more external “bands” towards the rear that are slightly larger than the bore (providing an
interference fit), but smaller than the combustion chamber (allowing easy loading). These bands, which
consist of different material(s) than the main body of the projectile, serve multiple functions such as
providing: a consistant loading position for the projectile when one of the bands axially comes to rest
against the forcing cone; a repeatable “shot-start force” that momentarily resists forward projectile motion
while the combustion process is just beginning; a seal to minimize the escape of combustion gases past the
projectile; a bearing surface to help keep the projectile centered while in-bore; and (if the bore is rifled) a
means of creating projectile spin for flight stability.
Driving Band
(also called the Rotating Band)
Obturator Band
Figure 3: Representative Large Caliber Projectile
1.3 INTERNAL BALLISTIC CYCLE
In preparation for firing the Gun, its breech is opened and a projectile is loaded into the rear of the Barrel,
passing through the combustion chamber and entering the bore. Next, a propelling charge is placed in the
combustion chamber, to the rear of the loaded projectile. When the propelling charge is ignited by an
energetic device in the breech, the propellant burns and produces gaseous products of combustion that
have very high pressure and temperatures.
kmFigure 4: Barrel Internal Pressure vs. Projectile Travel
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The propellant gases exert a force on the base of the projectile, and after they overcome the “shot-start
force” (provided by the driving band), it begins moving forward (“downbore”). Within milliseconds after
ignition, the projectile reaches the “muzzle” (the extreme forward end of the Barrel), where it is ejected at
high velocity towards its target. The gases that remain in the Barrel are then free to expand in the axial
direction, and they follow the projectile out the muzzle. Once they reach atmospheric pressure, the entire
internal ballistic cycle can be repeated. Typically a Gun Barrel is able to fire on the order of 102 or 103
rounds during its lifetime. The thermodynamic cycle for a Gun Barrel firing a projectile can be viewed
simply as a single-stroke engine. Figure 4 generically shows the nature of the pressure gradients when the
projectile is at various locations in the bore.
1.4 GUN BARREL MODES OF FAILURE
Gun Barrels have several potential modes of failure:
• If the maximum pressure developed in the combustion chamber or bore exceeds the elastic
strength pressure of the Gun Barrel at any axial location, “permanent bore expansion” can
result.
• In extreme cases, very high pressures can cause the Barrel walls to rupture. If the Barrel
material is brittle, the Gun Barrel can catastrophically “burst”. If the material is more
ductile, “gas leakage” will occur, a far more preferable outcome than bursting.
• Even at somewhat lower pressures, micro-cracks can form on the surfaces of the combustion
chamber, forcing cone, and/or bore due to the firing environment. Over many hundreds or
thousands of firing cycles, these can grow, coalesce, and eventually break through the Barrel
wall by a process known as “fatigue”.
• Finally, unacceptable loss of material can occur on the forcing cone or bore that is caused
either by the action of the hot gases passing over them at high velocity (a process called
“erosion” which typically has thermal, chemical and/or mechanical bore degradation
components) and/or by the projectile moving through the bore and interacting with the Barrel
walls (a process known as “wear”, which only has a mechanical bore degradation
component).
The likelihood of permanent bore expansion, bursting, and gas leakage can be mitigated by proper Gun
Barrel design practices. Fatigue is characterized by means of rigorous testing (a combination of firing and
lab simulations), the results of which are used to statistically determine the Barrel’s “fatigue life”. Even
though wear and erosion have separate causes, they are sometimes referred to simply as “wear” in large
caliber Gun Barrels. To maximize safety, the practice is almost always to design the Barrel so that its
“wear life” is less than its “fatigue life”.
1.5 GUN BARREL MATERIAL
In general, modern Gun Barrels are manufactured from low alloy steel forgings. Steel has been found to
be an excellent material for this application due to its balanced combination of high elastic yield strength,
surface hardness, ductility/fracture toughness, modulus of elasticity, and melting point. These properties
enable the resulting Gun Barrel to be resistant to all of the potential failure modes described above.
However, the desire for ever-increasing performance characteristics of weapon systems in recent years has
required that Guns operate at higher pressures, as well as increased firing rates and durations. Further, the
trend has been to utilize more energetic propellants that have higher flame temperatures and rates of bore
degradation. Designers of Gun Barrels are under constant pressure to reduce weight, resulting in thinner
walls, reduced thermal heat sink, and higher Barrel operating temperatures. All of the above tend to
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increase the rates of wear and erosion inside the Gun Barrel, despite gun steel’s excellent combination of
properties.
2.0 PRIMARY CAUSES OF GUN BARREL WEAR AND EROSION
2.1 Erosion
In most advanced Gun Barrels, the bore material exposed to hot combustion gases can undergo significant
thermal, chemical, and/or mechanical degradation. An unprotected steel bore is fully exposed to these hot
gases, and this can result in so-called “heat affected zones” (thermally altered layers) at and below the
bore surface; see Figure 5. This in turn leads to “heat checking” (which has an alligator skin appearance;
see Figure 6) and cracking of the bore surface, both of which increase with continued firing. In addition,
thermo-chemical firing damage is caused by hot gas-induced degradation of the fully exposed steel bore,
leading to gas-wash erosion.
(axial view) (radial / normal view)
Figure 5: Heat Affected Zone with Cracking Figure 6: Heat Checking
The bores of some very-high performance Guns are provided with a protective coating (see paragraph
6.3.2), and if it becomes damaged, the steel becomes partially exposed to the hot gases through cracks and
pits in the coating. General erosion mechanisms for these coated, advanced Gun Barrels also include
thermal, chemical, and/or mechanical components. As with an unprotected bore, thermal damage can
cause heat affected zones at and below any exposed substrate, leading to heat check cracking and platelet
micro-cracking of the bore coating and steel. Continued firing leads to bore coating shrinkage and heat
check crack widening. Thermochemical firing damage to the steel bore is caused by hot gas-induced
degradation of the partially exposed interface between the coating and substrate. This causes an
accelerated erosion rate leading to micro-pit formation and abrupt spalling, chipping, and stripping of the
protective coating.
2.2 Wear
Wear is a mechanical effect that is the result of interaction between the Gun Barrel and the projectile as
the latter traverses the bore. For example:
• If propellant gases escape past the rear band(s), they can create a sideward thrust on the projectile.
This, coupled with the projectile’s axial velocity and friction, can cause wear of the projectile
(“side body engraving”), the Gun Barrel, or both.
• For rifled Gun Barrels, if the projectile’s axial center of gravity (c.g.) is offset from its center of
rotation (c.r.), it can whirl or precess while moving downbore, causing some of the rifling lands to
wear selectively ("spiral wear") towards the muzzle end of the Barrel (see Figure 7 and Figure 12).
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Chromium-plated rifling land at 12 o’clock position Worn land at 6 o’clock position
on opposite side of bore.
Figure 7: Spiral Wear
• One of the bands towards the rear of the projectile is the so-called “rotating band” or “driving
band”, and it is generally made from a softer material than either the projectile body or Gun
Barrel. During loading and the initial stages of combustion, the driving band forcibly engraves
into the Barrel's rifling grooves. As the ballistic cycle continues, the driving band maintains an
interference fit that exerts a radial force on the bore. This can contribute to wear as the projectile
traverses the Gun Barrel, especially if the bore material has been thermally-damaged.
3.0 OBSERVATION AND MEASUREMENT OF WEAR AND EROSION
A variety of instruments are available to either observe or measure wear and erosion in large caliber Gun
Barrels:
3.1 Borescope
A photograph of a Borescope is shown at Figure 8:
Figure 8: Borescope
It is basically a long, thin pipe containing suitable lights, lenses, and mirrors that allows an inspector to
look closely at the bore surface over its entire length. The inspector typically describes his observations in
writing, but a capability is sometimes provided to also record the visual images permanently on videotape.
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The borescope’s output is often qualitative since depth of the wear/erosion cannot be ascertained. Despite
this limitation, an experienced inspector can make a meaningful determination of the Barrel’s condition
and suitability for continued use. Figure 9 shows a Borescope view for a newer rifled Barrel, and Figure
10 shows one for an unplated Barrel that has been fired many times.
Figure 9: Borescope View of a New Figure 10: Borescope View of a Worn
Rifled Barrel Rifled Barrel
Note the significant heat-checking (“alligator skin” pattern) of the worn Barrel. In Figure 11, the inspector
has installed a mirror that permits viewing at right angles to the bore. Again, heat checking is clearly
visible, which can contribute to both wear/erosion and fatigue.
Figure 11: Borescope View Normal to the Bore
3.2 Pullover Gage
This is a field measurement device that is capable of making a bore diameter measurement at a single axial
location. It is long enough to be inserted into the rear of the Barrel, passing through the combustion
chamber to a specified axial location in the bore. It is essentially an inside calliper, and the inspector
(usually a Soldier in the field) pulls a lever until the measurement surfaces touch the top and bottom of the
bore or rifling. The inspector then writes this diameter reading on a “record card” that accompanies the
Barrel throughout its service life.
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3.3 Stargage
This is a measurement device that is most frequently used in the Gun Barrel factory (for acceptance after
manufacturing) or at the Proving Ground (either for acceptance after initial proof firings or to measure
wear and erosion during developmental tests). It is as long as the entire Gun Barrel, enabling diametral
measurements to be taken at any axial location in the bore. Also an inside calliper, a stargage can be
inserted into the bore from either the breech end or the muzzle end in accordance with local practices. For
a smoothbore Barrel, all measurements are typically made with the contact pins oriented vertically (top-to-
bottom) over the entire bore length; then the process is repeated with the pins aligned horizontally (side-
to-side). For a rifled Barrel, the measurements can be made either by maintaining vertical or horizontal
alignment for the entire length of the bore (same as for a smoothbore) or by following the twist of a
specific pair of rifling lands. Either practice is acceptable, but once selected it is important to maintain the
same procedure throughout the Barrel’s life. After these diameter measurements have been recorded for
many axial locations, the results are graphed as a function of distance from the breech face of the tube. A
generic example is shown at Figure 12.
Figure 12: Generic Tube Wear Chart
3.4 Optical Bore-Mapping Systems
Each of the above inspection methods has its own set of benefits and drawbacks, but none gives a
complete visualization (qualitative and quantitative) of the bore’s condition. For this reason, bore-
mapping systems have been developed that are capable of making optical (usually laser) measurements of
the entire bore, then quantitatively displaying the bore condition using advanced imaging techniques.
Figure 13: Representative Bore-Mapping Output
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Figure 13, for example, shows a partial view of a rifled bore, forcing cone, and combustion chamber.
Note that the imaging computer has artificially “unrolled” the Barrel from its usual cylindrical shape into a
flat plane for easier observation. Optical bore-mapping systems have the added benefit of being able to
capture all data permanently on digital media. This technique provides a holistic understanding of the
wear and erosion pattern inside the Barrel.
4.0 EFFECTS OF GUN BARREL WEAR AND EROSION
Several potential failure modes for Gun Barrels were outlined earlier in paragraph 1.4. The first four
(permanent bore deformation, gas leakage, bursting, and fatigue) must be avoided since they pose a
potentially catastrophic threat to the gun crew and weapon system, as well as nearby personnel and
equipment. Wear and erosion is a more subtle (and perhaps tolerable) mode of failure since it usually
produces effects that have a gradual onset/progression and can for a time be compensated for. Some of
these effects are:
• Lowers muzzle velocity (decreases the projectile’s firing distance and/or terminal effectiveness);
• Causes inaccuracy (increases distance between intended aim-point and projectile impact-point);
• Increases dispersion (causes greater variation of projectile impact-points within a firing group);
• Produces unstable projectile flight (causes projectile to approach or strike the target incorrectly)
• Impairs proper fuze functioning (triggers at the improper time or not at all);
• Damages other sensitive projectile components (e.g., electronics, sensors, and guidance systems);
• Aggravates projectile driving band erosion and side-body engraving;
• Instigates surface defects in the bore and combustion chamber (hastens Barrel fatigue failure).
Although these effects are not usually hazardous to the crew operating the Gun, some can clearly be
dangerous to friendly personnel who are located downrange or near the intended target. Equally
importantly, if the intended target cannot be incapacitated, it remains a battlefield threat that is able to
return fire. And finally, the need to replace a large caliber Gun Barrel in the field represents a significant
life cycle cost on the order of 105 Dollars/Euros for each occurrence. For all these reasons, research and
development personnel devote significant effort not only to manage wear and erosion of Barrels in the
field, but also to mitigate this phenomenon in the first place.
5.0 MANAGING WEAR AND EROSION IN GUN BARRELS
Although wear and erosion is not a desirable phenomenon, the fact is that it initiates with the very first
round that is fired from a large caliber Gun Barrel; further, it continues progressively to the end of the
Barrel’s useful life. Despite that inevitability, the effects of wear and erosion are often not intolerable
until many rounds are fired (except in extremely high-performance Guns), and in many cases steps can be
taken to compensate for its presence.
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